1. Field of the Invention
The present invention relates to a work vehicle having an engine, wheels driven by power from the engine and a mower unit driven by the power from the engine.
2. Description of the Related Art
With such work vehicle as above-described, generally called a “lawn mower”, the wheels and the mower unit are driven by the power from the engine, so that the vehicle can effect a grass mowing (or lawn mowing) operation while vehicle is traveling.
In recent years, for a grass mowing operation, in particular, a lawn mowing operation, there is a need for finish with high quality. One condition to satisfy this need is optimization of the relationship between the driving speed of the mower unit and the traveling speed of the vehicle. For instance, a turf maintenance vehicle disclosed in the U.S. Pat. No. 5,394,678 includes a reel type mower unit and employs a control arrangement for optimizing the relationship between the rotational speed of the reel and the traveling speed. More particularly, this turf maintenance work vehicle includes a vehicle speed sensor for detecting a vehicle speed, a reel rotation sensor for detecting a rotational speed of the reel, a controller for calculating, as a control target, a target reel rotational speed based on a detected vehicle speed, and a further controller configured to compare a detected reel rotational speed and a target reel rotational speed and then execute a feedback control so that the actual reel rotational speed may follow the target reel rotational speed.
With this turn maintenance work vehicle disclosed in U.S. Pat. No. 5,394,678, the feedback control is executed such that even if the vehicle speed changes, there is provided a reel rotational speed suitable for this changed vehicle speed. However, in case the vehicle speed fluctuates with significant frequency and in significant range, this may provide adverse effect on the finish of the lawn.
Meanwhile, in the case of a tractor, a work vehicle similar to a lawn mower, there is implemented an engine control technique having an isochronous control mode for maintaining a constant engine rotational speed irrespectively of variation in the load applied to the engine and a droop control mode for varying the engine rotational speed according to varying load. In this, for a work accompanied by vehicle traveling (this will be referred to as “work-accompanied traveling” hereinafter) such as a vehicle traveling with hauling a cultivator for instance, the isochronous control mode is implemented, so that the engine rotational speed is maintained constant regardless of variation in the load. Whereas, for a normal vehicle traveling, the droop control mode is implemented.
For example, in the case of a work vehicle disclosed in the US 2009/0265082 A1, a controller is set with an engine control scheme (droop control) based on a first governor characteristics which is a torque curve according to which the engine rotational speed varies according to torque variation and an engine control scheme (isochronous control) based on a second governor characteristics which is a torque curve according to which the variation of the engine rotational speed for torque variation is smaller than the first governor characteristics or the engine rotational speed does not vary at all, irrespectively of torque variation. And, depending on operational positions of a hand accelerator lever and a setting switch, either one of the droop control scheme or the isochronous control scheme is implemented.
In order to maintain constant the cutter blade rotational speed of the mower unit during a grass mowing operation, implementing the above-described isochronous control scheme in a lawn mower is convenient. However, in such a case when the work-accompanied traveling and a no-work-accompanied traveling (i.e. traveling not accompanied by any work) are effected in repeated alternation, the traveling operability will deteriorate if the isochronous control scheme is implemented during a no-work-accompanied traveling also. To avoid this, a setting switch needs to be provided and a troublesome procedure will be required for the driver in which the driver operates the setting switch to select the isochronous control mode for an work-accompanied traveling or the droop control mode for a no-work-accompanied traveling.
In view of the state of the art described above, there is a need for a work vehicle having a control system with superior operability for maintaining the cutter blade rotational speed constant.